Electronic article surveillance systems are now commonly known for enabling the detection of the unauthorized removal of articles having some form of detectable marker secured thereto. Particular importance to the present invention, one type of such systems utilizes a radio frequency energy which is radiated into an interrogation zone along which articles having an electrically resonant circuit secured thereto must pass. Such a circuit absorbs energy from the field and reemits energy at its resonant frequency. Such reemitted energy is then detected by the receiver and an alarm produced as appropriate. For example, such systems are described in U.S. Pat. Nos. 3,810,147, 3,810,172, 3,740,742, and 4,023,167. In the systems there disclosed, two generally planar loop antennas are usually employed, one for transmitting and one for receiving, with each of the loop antennas generally being placed on opposite sides of an interrogation zone. Such a configuration will be recognized to complicate installation procedures.
It has further been noted that unshielded antenna configurations such as disclosed in the aforesaid patents are susceptible to external electrical noise often present in many commercial and industrial environments. Such noise is thus often picked up by the receiver antennas and confused with the low level signals produced by a marker circuit. In some cases, external noise may even directly interfere with, or load the transmitter antenna, such that improper transmit signals are produced. One attempt to minimize such interference effects is depicted in U.S. Pat. No. 4,251,808, the specification of which is incorporated herein by reference. In the system depicted therein, however, transmit and receive antennas are positioned on opposite sides of a exit way, with an electrostatic shield substantially enclosing only such antennas as include two or more twisted loops lying in a common plane. While the specification thereof notes that a single loop antenna may include an electrostatic shield wherein at least one discontinuity is provided to prevent current from circulating in the shield itself, the specification does not suggest the use of such a shield enclosing both an open loop and antennas having at least one pair of twisted loops.